1/3 october 1999 AN1162 application note power consumption comparison between 3v m29 series flash memories many products that use flash memories are battery powered and, therefore, they are sensitive to the pow- er consumption of each and every electronic part. the analysis of the power consumed by a flash memory requires knowledge of how often the memory is read because the read current is significantly different to the standby current. choosing the flash memory with the lowest overall power consumption is depen- dent on how the application uses the memory. flash memories from stmicroelectronics have very low read currents whereas amd and fujitsu tend to have lower standby currents. table 1 gives the maximum values for equivalent parts from each supplier. table 1. standby and read current supply requirements the figures show that the standby current is very much lower than the read current in all cases, by two or more orders of magnitude. it also appears, on first glance, that the stmicroelectronics part is going to use more power than the other two because the standby current is twenty times larger but the read cur- rent is only half. for many applications though it is the read current that is significant; not much of the time needs to be spent reading the memory before a considerable power saving can be made with the stmicroelectronics part. the current consumption of each memory over a period of time depends on how many times it is access- ed. a memory that is accessed 0% of the time will always consume the standby current whereas one that is accessed 100% of the time (at 5mhz) will always consume the read current. figure 1 shows a plot of the current consumption vs. the percentage of time that the part is accessed. figure 1. average current consumption vs. percentage time accessed manufacturer part standby current (max) read current at 5mhz (max) st m29w160b 100 m a8ma amd am29lv160b 5 m a 16ma fujitsu mbm29lv160 5 m a 17ma 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0ma 5ma 10ma 15ma 20ma st amd fujistu percentage of time part is accessed average current consumption ai02995
AN1162 - application note 2/3 because the read current is so much larger than the standby current the m29w160b clearly has a lower average current consumption than the am29lv160b and mbm29lv160 even when the flash is hardly used. clearly there must be a transition point where the consumption of the m29w160b starts to use more than the other two. figure 2 shows the region from 0% to 5%, the transition appears at just over 1%. figure 2. average current consumption vs. percentage time accessed (0% to 5%) in conclusion, the designer who wants to minimize the power consumption of flash memories will need to look deeper than the standby current. care should be taken to consider the read current, particularly in applications where the microprocessor reads look-up tables from the flash memory (e.g. m law and celp encoding/decoding for voice compression, sine wave tables for dtmf tone generation etc.) or is regularly accessing the memory. applications where the code is run directly from the microprocessor will only ever consume the standby current when the microprocessor is idle. st amd fujistu percentage of time part is accessed average current consumption ai02993 0% 0.5% 1% 1.5% 2% 2.5% 3% 3.5% 4% 4.5% 5% 0ma 0.2ma 0.4ma 0.6ma 0.8ma 1ma
3/3 AN1162 - application note if you have any questions or suggestion concerning the matters raised in this document please send them to the following electronic mail address: ask.memory@st.com (for general enquiries) please remember to include your name, company, location, telephone number and fax number. information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is registered trademark of stmicroelectronics a 1999 stmicroelectronics - all rights reserved all other names are the property of their respective owners. stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - u.s.a. http://www.st.com
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